As shown in FIG. 1, a prior art cooling device 1 is intended for use in a CPU (central processing unit) and is formed of a base 3. The base 3 is provided with a plurality of cooling fins 4. Now referring to FIG. 2, a prior art molding tool 7 is shown for manufacturing the cooling fins 8 by extrusion molding. In the process of making the cooling fins, the structural strength of the molding tool 7 must be taken into consideration in relation to the pressure of the extrusion molding. The ratio of height (H) and width (W) of the cooling fins 8, as well as the distance (D) between the two cooling fins 8 must be kept in an appropriate range. If the cooling fins 8 are too thin, the molding tool 7 is susceptible to damage in the extrusion molding. If the cooling fins 8 are excessively thick, there will be less cooling fins 8 to result in a reduction in the cooling area.